What's the Fender '63 Reissue Reverb Tank Tube Mod?

I've read a few comments here about a easy mod for the Fender '63 Reissue Reverb Tank involving tubes. What does this entail exactly? I know that there is a much more complicated mod which involves rewiring the circuit but this one sounds like something I could do myself.

Getting a good NOS 6K6 does eliminate some harshness. I heard contrary opinions on swapping the pre-amp tubes. My experience was positive on this. The stock tubes were forgettable. Some good NOS stuff in there added depth, richness to the tone. Past that (and the capacitor mod, which helped a tiny bit), best thing I did was swapping out the tank. The new tanks seem to be all over the place in quality and tone. Luck of the draw. Mine was harsh, even after the other mods. Tossed it and used another. Huge difference. Great sound now.

Hey thanks again zak! Sorry for all the questions. I'm just trying to do this correctly. You have been a great help as well as all the members here!!! I thank them for all their suggestions.
I just got my tank (1993 model) back from the shop. I bought it used from a Vintage Guitar Dealer on the East Coast. It arrived DOA. I took it to the shop and they had to replace the reverb pan. Needless to say he's getting the bill It works now. I guess UPS threw it around a bit. I don't know!

zak
No, it is a direct swap in the RI (6V6 to 6K6 that is...putting a 6V6 into an original tank designed for a 6k6 is NOT a good idea).

Ummm....tell that to Fender then!! The reason the RI sounds better with the 6K6 is that the RI IS the original design which was for a 6K6. You can interchange the 6K6/6V6 and it will work either way (6K6 in RI or 6V6 in vintage unit) but in my opinion it sounds better if you use a 6K6 in either one (the way it was intended by design) or you change a resistor to rebias it properly for the 6V6 which Fender for whatever reason decided not to do.

Bengal65
I've got a 6K6 NOS tube ready to go in my Reverb Unit. Does the unit have to be adjusted (bias?) when going from the 6V6 to the 6K6?

Not according to Fender or they would have biased it properly with a different value resistor when they shipped it with the 6V6!
The reason it sounds better w/ the 6K6 is that the RI circuit is set up for that tube because it copies the original...you ARE biasing it properly by putting in the correct tube.

Hi guys, I just did the "wood-shim-mod" to my brand new Reverb RI (made in U.S.A). And I'm amazed. The sound is much better now.

Before I did it I had to turn up the Dwell and the Mix almost fully to get some decent verb - and it was muddy and had barely any defined drip. So I opened up the front and unhooked the pan to find out that the input transducer was loose. It could be jiggled up and down easily. So I simply jammed a piece of a toothpick between the top of the coil and the metal part. The result is great, all the muddiness is gone and the drip is awesome!

Welcome Irv78. Exactly, it's a simple and very effective mod. I've found that pans with loose transducers have a sort of flutter echo quality, not a smooth decay. It's worth the trouble for all of you reverb tank owners to inspect your pan at some point, wiggle the transducers (the little coils on each end) and if they move at all, try jamming a tooth pick in the gap on the bottom between the transducers and the metal plates they sit in then break off the tooth pick leaving the shim. I'm not convinced about the epoxy but I may try it on one of my pans just to see.